Objects in conventional photography or detection reflect and may emit radiation of their own, and such detection of radiation includes all reflected and emitted radiation. Lock-in detection has been used with detectors wherein an object is externally stimulated and the lock-in detector detects only information of the image signal in synchronism with the reference stimulation signal.
In infrared (IR) or thermal imaging all objects emit thermal radiation of their own in addition to reflecting radiation of other objects. What is recorded in a thermograph is always a mixture of emitted and reflected radiation, some of which even comes from components of the camera, including lenses and their supporting structures. This problem is particularly severe in the 8-12 micrometer range, because it corresponds to the peak of black body radiation at room temperature. It is the same range of wavelength that is most relevant for nondestructive evaluation. In conventional scanned thermal wave imaging applications, this problem is overcome by the use of a lock-in analyzer synchronized to the source of the thermal waves. Without the lock-in technique, the IR video camera is capable of observing only very slow thermal phenomena. This limitation offsets the main advantage of the IR video camera, namely its high data-acquisition rate.
U.S. Pat. No. 4,652,757 issued Mar. 24, 1987 granted in the name of Carver discloses an infrared detector detecting a reflected probe beam that is modulated at the pump beam frequency produced by the pump laser. The lock-in amplifier allows the detector to process only the modulation frequency of the probe beam. The detected portion is only that which is modulated at the frequency of the pump beam. This is conventional analysis of a single signal with a commercially available lock-in amplifier.
U.S. Pat. No. 4,589,783 issued May 20, 1986 in the name of Thomas et al. discloses a thermal wave imaging system. A first laser beam heats the object and a second probe beam is deflected by the heated air above the object and is detected by a device mounted adjacent to the object and stored in an image memory under the control of central processor. This system also uses a single lock-in amplifier similar in fashion to that described with reference to the Carver patent.
The prior art only discloses receiving a single signal and using a lock-in amplifier to detect only the synchronous image. In general, the IR video camera is capable of observing only very slow thermal phenomena, despite the fact that the intrinsic band width of the camera is very broad. Additionally, without lock-in amplification all unsynchronous noise will be detected and displayed.